I. Field of the Invention
The present invention relates to communications. More particularly, the present invention relates to a novel and improved FM demodulator.
II. Description of the Related Art
Frequency modulation (FM) is an important modulation scheme used to facilitate transmission information from a transmitter to a receiver. An exemplary application which utilizes FM modulation is a cellular communication system which conforms with the Advanced Mobile Phone System (AMPS) standard. In FM modulation, the frequency of a carrier sinusoid is varied in accordance with a modulating signal which contains the information. The FM signal can be expressed as: EQU y(t)=cos2.pi.(f.sub.c +A.sub.o m(t))t!, (1)
where y(t) is the FM signal, f.sub.c is the frequency of the carrier sinusoid, A.sub.o is the gain which controls the amount of frequency deviation, and m(t) is the modulating signal. FM modulation can be implemented in one of numerous methods. For example, FM modulation can be achieved by providing the control voltage of a voltage controlled oscillator (VCO) with the modulating signal. The frequency modulated signal from the VCO comprises the FM signal which is then conditioned and transmitted to the receiver.
At the receiver, the FM signal can be demodulated in one of numerous methods. For analog FM demodulation, an FM discriminator can be used to convert the frequency modulated FM signal into an amplitude modulated signal which can be detected with an envelop detector. Alternatively, analog FM demodulation can be performed by a phase locked loop (PLL) which locks a VCO within the PLL to the received FM signal. Once the PLL locks to the FM signal, the control voltage to the VCO comprises the estimate of the modulating signal. FM detection using FM discriminator and PLL are commonly used in the art.
The received FM signal can also be demodulated digitally using a quadrature receiver such as the receiver disclosed in U.S. patent application Ser. No. 08/987,308, entitled "RECEIVER WITH SIGMA-DELTA-CONVERTER"filed Dec. 9, 1997, assigned to the assignee of the present invention and incorporated by reference herein. In an exemplary digital FM demodulation scheme, the quadrature I and Q baseband samples from the receiver are provided to an arc-tangent lookup table to convert the baseband samples into phase information. The phase information is then differentiated to produce the demodulated output.
The analog and digital FM demodulation schemes of the prior art require additional circuitry which can increase the complexity of the receiver. The additional circuitry increases cost, reduces reliability, and increases the circuit size and weight. These factors are important considerations for receivers which are produced in high quantity. Furthermore, the additional circuitry consumes power which can decrease the operating time of a receiver used in mobile applications, such as a cellular telephone. A new scheme is needed to perform FM demodulation while reducing circuit complexity and minimizing power consumption.